The invention relates to a heating device having a heating space which is accessible for objects to be heated inside said space and to which space thermal energy from at least one heat source can be supplied.
Devices of this type are known and are used for a great variety of purposes. They are used, for example, to dry lacquered objects, to bake layers of (ceramic) material, to keep molten materials at the desirable temperature during their transport through a duct so as to prevent solidification of the liquid in the transport duct, to keep molten glass at the desirable temperature during drawing glass in the form of tubes or rods, to heat wire material, for example tungsten, before it enters the die, and so on.
In practice the heat sources are usually constituted by electric resistance wires heated by the passage of current (see, for example, Dutch Pat. No. 34,607 and U.S. Pat. No. 3,063,268) or gas burners (see, for example, U.S. Pat. No. 3,253,898 and the published French patent application No. 2,007,949).
The known heating devices have several drawbacks. In many manufacturing processes it is of great importance that the temperature in the heating space should have the same value everywhere, in particular when, in addition to the thermal treatment, the objects have to be subjected to other treatments in the heating space, for example, the bending of glass sheets, described in the above U.S. Pat. No. 3,253,898. A non-uniform temperature in the heating space results in stresses in the material, as a result of which fracture and a high reject percentage occur, which therefore is undesirable. Isothermal surroundings are also of great importance in all kinds of measurements, for example in the calibration of thermocouples in a calibrating furnace. In order to obtain the heating space in heating devices which are provided with electric resistance wire heating as isothermal as possible, the wires are wound around the heating space throughout the length of the device, and thermally insulated from the atmosphere as readily as possible. In particular in the case of long transit spaces, this makes the construction of the heating device complicated and expensive. In addition, at the beginning and at the end of the transit space, the thermal losses are always larger than in the center and therefore the temperatures are lower at those regions. In order to compensate for this as much as possible, new structural measures are required, for example, winding the resistance wire at the beginning and at the end of the transit space with a smaller pitch than in the
In heating devices employing gas heating, as described in the above U.S. Pat. No. 3,253,898, the isothermal character of the transit space is endeavoured by arranging a large number of gas burners in said space. This also makes the device complicated and expensive. The same applies to heating devices in which a large number of filament lamps, especially those constructed as infrared radiators, are mounted. Furthermore, a separate control for each gas burner and infrared radiator, respectively, is required for the temperature adjustment of the heating space.
It is the object of the present invention to provide a heating device of a simple and inexpensive construction and of which the heating space is isothermal during operation throughout its dimensions.